In order to make image information visible by means of an electrophotographic method an electrostatic latent image is firstly formed on a photoconductor drum of an image-forming apparatus, then it is developed with a toner and then transferred to e.g. transfer paper, and the transferred image is fixed by e.g. heat to form a visible image. Heretofore, a toner commonly employed has been produced by a so-called melt-kneading pulverization method i.e. a method wherein an antistatic agent, a magnetic material, etc., are dry-mixed to a binder resin such as a styrene/acrylate copolymer or a polyester produced by various methods and a colorant, as the case requires, and the mixture is then melt-kneaded by e.g. an extruder, followed by pulverization and classification.
In recent years, a low temperature fixing property and reduction of the particle size of the toner have been required more than ever in order to accomplish a high image quality and high speed desired for printers or copying machines. As a method to improve the melt kneading pulverization method from such a viewpoint, a suspension polymerization method wherein a mixed liquid containing a polymerizable monomer, a colorant and a polymerization initiator is suspended or dispersed in an aqueous medium to form droplets of a suitable size, followed by polymerization to obtain toner particles, or an emulsion polymerization flocculation method wherein a colorant and optionally an antistatic agent, etc., are added to a dispersion of primary particles of a polymer obtained by emulsion polymerization to carry out flocculation and aging to obtain toner particles, has been proposed and practically employed. When a toner is obtained by such a production method so-called a polymerization method, control of the particle size of the toner is easy and it is possible to obtain a toner having a small particle size and a narrow particle size distribution. Further, in the case of the polymerization method, no pulverization step is required, and it is possible to produce a toner using a binder resin having a low softening point, and it is possible to obtain a toner with a high resolution and excellent in the low temperature fixing property.
On the other hand, even such a toner having the low temperature fixing property improved has had a problem that the temperature range for fixing is not necessarily secured, and if fixing is attempted at a high temperature, offset is likely to result. To prevent such offset, it is common to add wax to the toner. With a release agent such as wax, the effect to suppress offset (the releasing effect) can be expected depending upon the amount. However, if wax is incorporated in a large amount relative to the toner, such a phenomenon is likely to occur that wax leaches out on the toner surface, or that the wax is liberated from the toner. As a result, there has been a problem such that the toner or the liberated wax soils various portions of the image-forming apparatus; or the toner itself is likely to be degraded by a phenomenon such that the toner undergoes blocking during the storage or an external additive on the toner surface is gradually embedded in the toner.
As a method for preventing wax in the toner from leaching out on the toner surface, a method was, for example, proposed wherein resin particles containing wax were coated with a resin containing no wax (e.g. Patent Document 1). However, when this method was employed, there was a case where no adequate release effect by wax was obtainable at the time of fixing. Further the affinity between the binder resin and wax was not good, whereby it was not possible to sufficiently prevent leaching out or liberation of wax on the toner surface.
Patent Document 1: JP-A-2002-82487
Further, in the slurry of toner particles obtained by a polymerization method, impurities such as an emulsifying agent, a suspension stabilizer, etc are present. If these impurities will remain as they are in a developer, a problem such as deterioration in the powder characteristics due to absorption of moisture will occur. Therefore, it is usually required to remove such impurities in the slurry by a step of e.g. filtration during the production of the toner. However, if it is attempted to incorporate wax in a large amount in the toner, there has been a problem that due to an influence of wax leached out on the toner surface or wax liberated from the toner clogging of the filter membrane is likely to occur during the filtration step, whereby the productivity of the toner has been substantially deteriorated.
A method has been proposed wherein the compatibility of wax and the binder resin is increased by using a binder resin obtainable from a polymerizable monomer containing a long chain alkyl acid ester and/or methacrylic acid ester, to make them readily compatible with each other (e.g. Patent Document 2). However, if this method is employed, the binder resin and wax tend to be compatible too much, whereby the diffusivity of wax in the binder resin tends to be so high that wax is likely to reach and leaches out on the toner surface in a short time. Further, if wax will not remain in the form of fine crystalline domains in the binder resin and will be molecularly dissolved, the binder resin tends to be plasticized. Therefore, there has been a drawback that inclusion of wax in the toner is deteriorated, whereby blocking resistance tends to be poor, or it becomes difficult to simultaneously satisfy the low temperature Fixing property and the high temperature offset.
Patent Document 2: JP-A-7-301949
Here, in a case where a toner is to be produced by an emulsion polymerization flocculation method, a method is known wherein a long chain alkyl acid ester and/or a methacrylic acid ester is employed. For example, Patent Document 3 discloses a toner comprising a binder resin containing a crystallizable resin employing stearyl (meth)acrylate and at least one member of resins having a contact angle with water smaller than the binder resin. However, also such a method is not sufficient for prevention of leaching out of wax on the toner surface.
Patent Document 3: JP-A-2002-108018
As described above, it has not been known how i is possible to add wax in a large amount in a toner and to prevent leaching out of wax on the toner surface or liberation of wax from the toner, and such has not been accomplished by prior art.